As a rolling bearing unit for rotatably supporting wheels of a vehicle with respect to a suspension device, for example, Patent Document 1 discloses a structure of a rolling bearing unit for supporting wheels as illustrated in FIG. 18. In a rolling bearing unit 1 illustrated in FIG. 18, an outer ring 2 serving as a stationary side race ring and a hub 3 serving as a rotary side race ring are arranged to be concentric with each other. Then, each set of multiple balls 6 and 6 respectively serving as rolling elements is arranged in each double row between double row outer ring races 4 and 4 disposed on an inner peripheral surface of the outer ring 2 and respectively serving as the stationary side races and double row inner ring races 5 and 5 disposed on an outer peripheral surface of the hub 3 and respectively serving as the rotary side races. The respective balls 6 and 6 are held by respective retainers 7 and 7 so as to be rollable. This configuration allows the hub 3 to be rotatably supported on an inner diameter side of the outer ring 2. In order to rotatably support the wheels with respect to the suspension device, a stationary side flange 8 disposed on the outer peripheral surface of the outer ring 2 is fixedly screwed to the suspension device, and the wheels and a braking rotor such as a disc rotor is fixedly screwed to a rotary side flange 9 formed on the outer peripheral surface of an outboard end portion of the hub 3 (an outboard side is referred to as a left side in each drawing which is an outer side in a width direction of a vehicle in a state of being assembled in the vehicle. In contrast, a right side in each drawing which is an inner side in the width direction of the vehicle is referred to as an inboard side).
Both end openings of a bearing internal space 10 in which the respective balls 6 and 6 are installed between the inner peripheral surface of the outer ring 2 and the outer peripheral surface of the hub 3 are respectively closed over an entire periphery by a seal ring 11 and a combination seal ring 12. The seal ring 11 which closes an outboard end side opening of the bearing internal space 10 is formed by reinforcing an elastic member 14 made of an elastomer such as rubber using a metal insert 13 made of a metal plate and by configuring an entire body to have an annular shape. In addition, the elastic member 14 has multiple (three in an illustrated example) seal lips 15a, 15b and 15c. Then, in a state where the metal insert 13 is tightly and internally fitted to an outboard end portion on the inner peripheral surface of the outer ring 2, distal end edges of the respective seal lips 15a, 15b and 15c are respectively brought into sliding contact with an inboard side surface of the rotary side flange 9 or an outer peripheral surface of an intermediate portion of the hub 3, over the entire periphery.
In addition, the combination seal ring 12 is formed by combining the slinger 16 and the seal ring 17. The slinger 16 is configured so that an entire body is annularly stamped in an L-shape in cross-section by bending a metal plate, and is configured to have a rotary side cylindrical portion 18 and a rotary side annular portion 19 which is bent radially outward from an inboard end edge of the rotary side cylindrical portion 18. This slinger 16 is fixed to the hub 3 by tightly and externally fitting the rotary side cylindrical portion 18 to the inboard end portion of the hub 3 (a hub body and inner ring configuring the hub 3). In addition, the seal ring 17 includes a metal insert 20 made of a metal plate and an elastic member 21. The elastic member 21 includes multiple (three in an illustrated example) seal lips 22a, 22b and 22c. In addition, the metal insert 20 is configured so that an entire body is annularly stamped in an L-shape in cross-section by bending a metal plate, and is configured to have a stationary side cylindrical portion 23 and a stationary side annular portion 24 which is bent radially inward from an outboard end edge of the stationary side cylindrical portion 23. The seal ring 17 including this metal insert 20 is fixed to the outer ring 2 by tightly and internally fitting the stationary side cylindrical portion 23 to the inboard end portion of the outer ring 2. In addition, in this state, a distal end edge of the seal lip 22a is brought into sliding contact with an outboard side surface of the rotary side annular portion 19, and distal end edges of the respective seal lips 22b and 22c are respectively brought into sliding contact with an outer peripheral surface of the rotary side cylindrical portion 18, over the entire periphery.
There is also a case where out of both axial side surfaces of the rotary side annular portion 19 of the slinger 16 configuring the above-described combination seal ring 12, an annular encoder 25 is fixedly attached to a side surface (inboard side surface) opposite to a bending direction of the rotary side cylindrical portion 18 over the entire periphery, as illustrated in FIG. 19A. The encoder 25 is made of a permanent magnet such as a rubber magnet or a plastic magnet in which magnetic powder is dispersed in a high polymer material such as rubber or synthetic resin and an entire body is formed to have an annular shape. The encoder 25 is magnetized in the axial direction. The magnetized directions vary alternately and at equal intervals in a circumferential direction. Accordingly, S poles and N poles are arranged alternately and at equal intervals in the circumferential direction on an inboard side surface of the encoder 25 which is a detection target surface. It is possible to measure a rotation speed of the wheels rotating with the hub 3 by causing a detection portion of a rotation detection sensor (not illustrated) to oppose the detection target surface of the above-described encoder 25. Then, the measured signal indicating the rotation speed of the wheels is used in controlling a travelling stabilization device of a vehicle such as an anti-lock braking system (ABS) and a traction control system (TCS). A structure and an operation of this rotation speed detection device are known in the related art. Accordingly, since these are not related to the gist of the present invention, illustration and detailed description will be omitted.
In contrast, when it is not necessary to detect the rotation speed of the vehicle, or when it is possible to detect the rotation speed by using a different portion, it is not necessary to dispose an encoder in the slinger 16 as illustrated in FIG. 19B.
In any case, two axial end openings of the bearing internal space 10 are closed by the seal ring 11 and the combination seal ring 12, respectively over the entire periphery. This manner prevents leakage of grease which is present inside the bearing internal space 10, and prevents foreign matters such as moisture and dust which are present in an external space from permeating the bearing internal space 10.
In order to ensure durability of the rolling bearing unit 1 for supporting the wheels, it is important to ensure sealing performance of the seal ring 11 and the combination seal ring 12. On the other hand, the seal ring 11 and the combination seal ring 12 are used under severe conditions where muddy water is sprayed thereon when in use. In particular, in a case of the combination seal ring 12, there is a possibility that moisture of the muddy water which has permeated the seal internal space 26 surrounded by the slinger 16 and the seal ring 17 may be evaporated and only a solid portion (mud) may be accumulated inside the seal internal space 26. In a case of the structure in the related art which is illustrated in FIGS. 18, 19A and 19B, a volume is large in a space close to the outside 28 which is present in a portion further radially outward than the seal lip 22a present in the outermost diameter side within the seal internal space 26 and which communicates with the external space via the labyrinth seal 27. Accordingly, an amount of the solid portion accumulated inside the space close to the outside 28 is likely to increase. Then, there is a possibility that the solid portion accumulated inside the space close to the outside 28 may also adhere to the seal lip 22a present in the outermost diameter side and may hinder a movement of the seal lip 22a. As a result, there is a possibility that a portion of the distal end edge of the seal lip 22a may be lifted from an outboard side surface of the rotary side annular portion 19 of the slinger 16 and the foreign matters such as the muddy water may more deeply permeate the seal internal space 26 through the lifted portion. This phenomenon becomes particularly conspicuous in the muddy water containing clay soil which is likely to be condensed by evaporation of moisture.
If a clearance dimension (width dimension in the radial direction) of the labyrinth seal 27 is decreased, it is possible to reduce the foreign matters such as the muddy water permeating the space close to the outside 28. However, there is a limit in decreasing the clearance dimension. That is, if the clearance dimension of the labyrinth seal 27 is greatly decreased, there is a possibility that a moment load applied during the turning of a vehicle may cause the outer ring 2 and the hub 3 to be relatively tilted, a distance of the labyrinth seal 27 may be shortened, and an end edge of the rotary side annular portion 19 and an inner peripheral surface of the stationary side cylindrical portion 23 may come into intense (strong) contact with each other. Therefore, in the structure in the related art which is illustrated in FIGS. 18, 19A and 19B, it is difficult to greatly decrease the clearance dimension of the labyrinth seal 27.
As a structure for improving the sealing performance of the combination seal ring. Patent Document 2 discloses a structure in which a distal end edge of a second seal lip configuring a second seal ring externally fitted to an outer ring is brought into sliding contact with a side surface which is an opposite side to the seal ring within the rotary side annular portion of the slinger, over the entire periphery. According to this structure, it is possible to sufficiently prevent the foreign matters from permeating the seal internal space. However, the number of components is increased and the cost is increased. In addition, there is a possibility that an installation space for disposing the second seal ring is increased and design freedom may be limited in order to prevent interference with other articles disposed adjacent to the rolling bearing unit with the combination seal ring. Furthermore, as the second seal lip, those which have rigidity equivalent to that of the seal lip originally disposed on the combination seal ring side are disposed by being added to the originally disposed seal lip. Therefore, there is a disadvantage in that a sliding contact portion is increased and drag torque (rotational dynamic torque) is increased.